A fluidized catalytic cracking (FCC) process is a process that cracks higher molecular weight hydrocarbons down to gasoline and liquefied petroleum gas (LPG) range hydrocarbons. The FCC process is carried out by contacting hydrocarbonaceous feed material such as vacuum gas oil, residual crude, or another source of relatively high boiling hydrocarbons with a catalyst made up of finely divided or particulate solid material in an elongated conduit. Contact of the feed with the fluidized catalyst particles catalyzes the cracking reaction while coke is deposited on the catalyst. Catalyst exiting the reaction zone is spoken of as being “spent”; i.e., partially deactivated by the deposition of coke upon the catalyst. Spent catalyst is conventionally transferred to a stripper that removes adsorbed and entrained hydrocarbon gases from catalyst and then to a regenerator for purposes of removing the coke by oxidation with an oxygen-containing gas. Regenerated catalyst is returned to the reaction zone. Oxidizing the coke from the catalyst surface releases a large amount of heat, a portion of which leaves the regenerator with the regenerated catalyst.
Spent catalyst still has catalytic activity. Prolonged contact between spent catalyst and cracked product gases can allow overcracking of desired products and additional coke deposition, thereby diminishing the recovery of desired product. Spent catalyst and gas products exiting the reactor conduit typically enter into a voluminous reactor vessel in which they may reside for prolonged times before separation, thereby allowing additional cracking to occur. Separation devices at the discharge end of the riser conduit have been used to quickly separate much of the catalyst and gaseous product.
Cyclones for separating particulate material from gaseous materials exiting the riser conduit or collected from the large reactor vessel are well known to those skilled in the art of FCC processing. Cyclones usually comprise an inlet that is tangential to the outside of a cylindrical vessel that forms an outer wall of the cyclone. In the operation of an FCC cyclone, the entry and the inner surface of the outer wall cooperate to create a spiral flow path of the gaseous materials and catalyst that establishes a vortex in the cyclone. The centripetal acceleration associated with an exterior of the vortex causes catalyst particles to migrate towards the outside of the barrel while the gaseous materials enter an interior of the vortex for eventual discharge through an upper outlet to a gas conduit. The heavier catalyst particles accumulate on the side wall of the cyclone barrel by centrifugal force and eventually drop to the bottom of the cyclone and out via a lower outlet into a catalyst bed.
The catalyst bed is typically fluidized to facilitate entry of the catalyst into a stripper vessel. The reactor vessel contains a large volume of empty space in which catalyst can become entrained with gaseous product that has not yet exited the reactor vessel. Entrainment can occur when catalyst is being transferred between separator stages, transferred from the cyclone dipleg into the catalyst bed and fluidized in the catalyst bed. Typically, the reactor vessel is purged with an inert gas, such as steam, to activate low fluidization zones and quicken the entry of product gases with entrained catalyst particles into the cyclones to eventually exit from the reactor vessel.
Gas conduits from multiple cyclones may all have outlet ends in a plenum located inside or outside the top of the reactor vessel. Product gases from the gas conduits exiting the cyclones collect in the plenum and eventually exit the reactor vessel or the plenum to be further refined.
Accordingly, it is an object of the present invention to improve the efficiency of separating particulate solids from vapors in an FCC unit.